Candida albicans is an opportunistic pathogen capable of causing debilitating mucosal infections as well as life-threatening systemic infections. Individuals infected with Human Immunodeficiency Virus (HIV) are particularly susceptible to Candida infections, with 90% of this population experiencing oropharyngeal candidiasis at some stage of the disease. C. albicans is ubiquitously present in the natural microflora of the body and its success as both a commensal and a pathogen relies on its ability to rapidly adapt to changes in host physiology. This proposal will establish the role of phenotypic switching in C. albicans biology, with emphasis on the role of switching in promoting colonization and infection of distinct sites in the immunocompromised host. Phenotypic switching refers to an epigenetic phenomenon in which cells undergo a reversible and heritable change in colony morphology and gene expression. In C. albicans, elevated rates of phenotypic switching are associated with increased invasive growth and drug resistance, particularly in HIV-infected individuals, making an understanding of the mechanism(s) of switching of paramount importance. Experiments outlined in this proposal will address the mechanism of phenotypic switching between white and opaque forms of C. albicans. The white-opaque switch plays a pleiotropic role in C. albicans biology, directing tissue specificity, virulence, and biofilm formation, yet the molecular mechanism of white-opaque switching is only now beginning to be elucidated. Preliminary experiments indicate that multiple, apparently diverse, factors impact the rate of white-opaque switching indirectly by their effect on cellular growth. The unexpected connection between phenotypic switching and cell growth will be further investigated in Aim 1, as it represents a novel mechanism by which the cell can adapt to environmental stress. The second Aim of the proposal will identify novel regulators of the white-opaque switch using an overexpression screen of transcription factors. It is expected that the construction of an overexpression library will also prove a useful genetic tool for many researchers in the field. Finally, the third Aim of the proposal will establish the temporal and spatial regulation of white- opaque switching in the mammalian host using RIVET (recombinase in vivo expression technology). Thus, taken together, the proposed studies will determine both the in vitro regulation of the white-opaque switch by environmental and host factors, as well as defining the in vivo niche where switching occurs. The completion of these studies will therefore shed light on how phenotypic switching is used by C. albicans to promote adaptation to the host and help avoid clearance by the immune system. PUBLIC HEALTH RELEVANCE: Candida albicans is the most common fungal pathogen in humans, causing both debilitating mucosal infections and life-threatening systemic infections. Our research focuses on the mechanisms used by C. albicans to generate phenotypic diversity and how these processes promote survival and infection in the mammalian host. The research also addresses how C. albicans is able to switch from growing as a harmless commensal to an opportunistic pathogen that can invade virtually all the organs in the body.